Giovanna Della Porta

Carbonate build-ups in lacustrine, hydrothermal and fluvial settings: comparing depositional geometry, fabric types and geochemical signature

Abstract Carbonate build-ups in lakes, hydrothermal and fluvial settings are characterized by distinctive geometry, spatial distribution, fabrics and geochemical signature but also by some comparable features. Lake margin bioherms form continuous belts for hundreds of metres to kilometres, subparallel to shorelines. Sublacustrine spring mounds are spaced at hundreds of metres to kilometres and aligned along faults. Hydrothermal travertine mounds and aprons with planar clinoforms or terraced slopes are controlled by faults, thermal water discharge and substrate topography. Fluvial tufa barrages, cascades and terraced slopes are controlled by climate, vegetation and substrate gradient. The wide spectrum of carbonate microfabrics ranges from clotted peloidal micrite and laminated boundstone to crystalline dendrite cementstone. Non-marine carbonate microfabrics cannot be linked to specific depositional environments, and are not deterministic proxies for the interpretation of build-up architecture. Microfabric associations can be indicative, but not exclusive, of specific depositional environments and geometry. Stable isotope geochemistry is a useful tool to distinguish between hydrothermal, karstic freshwater and evaporative lake carbonates. Carbonate precipitation results from a continuum of abiotic and biologically influenced/induced processes in settings where carbonate supersaturation is largely driven by physico-chemical mechanisms and microbial biofilms, even if acting as passive low-energy surface sites for nucleation, are widely present.

The role of petrography on the thermal decomposition and burnability of limestones used in industrial cement clinker

The present research examines the influence of the petrographic features on the thermal decomposition and burnability of three limestones, the main raw materials for Portland cement-making. A detailed characterisation of the limestones has been performed by means of optical microscopy and X-Ray Powder Diffraction. The carbonate thermal decomposition was conducted under isothermal conditions by means of in situ High Temperature X-Ray Powder Diffraction and the heated samples were further investigated by Scanning Electron Microscopy. Three kiln feeds were then prepared and submitted to burning trials and the temperature of occurrence of the main clinker phases was investigated as well as the content of the uncombined CaO in the heated samples was determined by using the Franke method. The results attest that the microfabric, a combination of depositional and diagenetic features, drives the kinetics of the thermal decomposition of the selected limestones as well as it appears to influence the temperature of crystallisation of the main clinker phases and the uncombined CaO content in the final clinker. In particular, the limestone with the lowest micrite to sparite ratio (1) exhibits the lowest Apparent Activation Energy (Ea) value and the highest rate of calcination and (2) requires a lower temperature for observing the clinker phases crystallisation and has the lowest content of uncombined CaO in the final clinker, thus reflecting a high burnability of the limestone.

DEPOSITIONAL ARCHITECTURE, FACIES CHARACTER AND GEOCHEMICAL SIGNATURE OF THE TIVOLI TRAVERTINES (PLEISTOCENE, ACQUE ALBULE BASIN, CENTRAL ITALY)

Facies character, diagenesis, geochemical signature, porosity, permeability, and geometry of the upper Pleistocene Tivoli travertines were investigated integrating information from six borehole cores, drilled along a 3 km N-S transect, and quarry faces, in order to propose a revised depositional model. Travertines overlie lacustrine and alluvial plain marls, siltstones, sandstones and pyroclastic deposits from the Roman volcanic districts. In the northern proximal area, with respect to the inferred hydrothermal vents, travertines accumulated in gently-dipping, decametre-scale shallow pools of low-angle terraced slopes. The intermediate depositional zone, 2 km southward, consisted of smooth and terraced slopes dipping S and E. In the southernmost distal zone, travertine marshes dominated by coated vegetation and Charophytes interfingered with lacustrine siltstones and fluvial sandstones and conglomerates. Travertine carbon and oxygen stable isotope data confirm the geothermal origin of the precipitating spring water. The travertine succession is marked by numerous intraclastic/extraclastic wackestone to rudstone beds indicative of non-deposition and erosion during subaerial exposure, due to temporary interruption of the vent activity or deviation of the thermal water flow. These unconformities identify nine superimposed travertine units characterized by aggradation in the proximal zone and southward progradation in the intermediate to distal zones. The wedge geometry of the travertine system reflects the vertical and lateral superimposition of individual fan-shaped units in response to changes in the vent location, shifting through time to lower elevations southward. The complexity of the travertine architecture results from the intermittent activity of the vents, their locations, the topographic gradient, thermal water flow paths and the rates and modes of carbonate precipitation.

Intact seismic-scale platforms and ramps in the Lower to Middle Jurassic of Morocco: Implications for stratal anatomy and lithofacies partitioning

The Jurassic carbonate platforms of the central High Atlas in Morocco are well known for several high-quality outcrops. In the central High Atlas, there are two complementary locations that offer critical lessons for our understanding of Jurassic carbonate system evolution in extensional basins: a Lower Jurassic high-relief, carbonate platform with steep slopes that developed on the footwall of a rotating fault block in an active half-graben (Djebel Bou Dahar [DBD]) and an upper Lower to Middle Jurassic low-angle prograding carbonate ramp rich in ooids (Amellago ramp [AR]). The DBD and AR outcrops provide superbly exposed, structurally intact, and fully accessible platform to basin transects. They provide valuable analogs for depositional geometries at reservoir and seismic scales that are highly relevant for hydrocarbon exploration and production. The DBD serves as an analog for isolated carbonate platforms developed in rift basins (particularly synrift carbonate platforms with a coral calcareous sponge and microbial boundstone facies belt in the upper slope and margin). The AR provides one of the rare examples whereby a large-scale oolitic ramp can be examined in great detail, providing an analog for a range of oolitic reservoirs, mostly Mesozoic.

The Sierra del Cuera (Pennsylvanian microbial platform margin) in Asturias, north Spain

ABSTRACT Stratal geometry and lithofacies architecture from carbonate outcrops provide critical information for subsurface prediction of rock types, their spatial distribution, and reservoir quality. Such observations guide and support exploration, appraisal, development, and production strategies. The Pennsylvanian Sierra del Cuera (SDC) carbonate platform outcrop of the Cantabrian Mountains in north Spain offers such information through a seismic-scale cross section of a microbial, unrimmed, and high-relief platform to basin transect, deposited during an icehouse period. The SDC outcrops provide alternative platform margin models to those developed from coral–algal-dominated Neogene carbonate systems and serve as direct analogs for supergiant fields such as Tengiz, Kashagan, and Karachaganak in the Pricaspian Basin of Kazakhstan and potentially for other high-relief platforms with microbial margins. Key features of the SDC platform are (1) a flat-topped platform with a rollover into a steep (30°–40°) and high-relief (600–850 m [1960–2790 ft]) depositional slope and (2) the presence of a microbial boundstone factory from the platform break to nearly 300 m (984 ft) paleowater depth on the slope, stabilized by pervasive marine cementation. The SDC represents a nonactualistic type of carbonate platform margin where the microbial carbonate factory on the upper slope controlled the rate of progradation instead of the platform top–sourced sediment input. These features have significant implications for the interpretation of the evolution and controlling factors of carbonate depositional systems in outcrops and in the subsurface.